Luca Zanini

Low dimensional neutron moderators for enhanced source brightness

In a recent numerical optimization study we have found that liquid para-hydrogen coupled cold neutron moderators deliver 3-5 times higher cold neutron brightness at a spallation neutron source if they take the form of a flat, quasi 2-dimensional disc, in contrast to the conventional more voluminous shapes used by now. In the present paper we describe a simple theoretical explanation of this unexpected behavior, which is based on the large difference in para-hydrogen between the values of the scattering mean free path for thermal neutrons (in the range of 1 cm) and its much larger equivalent for cold neutrons. This model leads to the conclusions that the optimal shape for high brightness para-hydrogen neutron moderators is the quasi 1-dimensional tube and these low dimensional moderators can also deliver much enhanced cold neutron brightness in fission reactor neutron sources, compared to the much more voluminous liquid D2 or H2 moderators currently used. Neutronic simulation calculations confirm both of these theoretical conclusions.

Optimization of moderators and beam extraction at the ESS

All instruments at the European Spallation Source (ESS), Lund, Sweden, are served by a carefully optimized moderator assembly, providing world-leading performance and excellent flexibility and upgradeability.

General use of low-dimensional moderators in neutron sources

The European Spallation Source (ESS) will use low-dimensional moderators for cold and thermal neutron production. Low-dimensional moderators deliver higher slow neutron brightness compared to conventional volume moderators. The concepts developed at ESS could be used in reactors and compact neutron sources to reach large increases in neutron flux to the experiments, compared to common practice. For reactors, the smaller volume of the moderators allows positioning them at the optimum flux region. For compact neutron sources, the reduced heat deposition and lower radiation environment allows for particularly efficient implementation of the ESS concepts. Using tube moderators in a compact source would allow reaching a higher brightness per unit yield at least a factor of 4, with respect to a spallation source.

The neutron moderators for the European Spallation Source

The European Spallation Source will have 42 beam ports as a grid available for a variety of instruments, mostly neutron scattering experiments. Bi-spectral extraction for thermal and cold neutrons must be available to all the beam ports. The moderator design to deliver such neutron beams was driven by the low-dimensional moderator concept. The adopted design, consisting of one flat (3 cm high) moderator placed above the spallation target was considered valid for the initial instruments suite. ESS will however have a beam port system designed such that it will be possible to extract neutrons from moderators above and below the target. With all initial instruments pointing to the top moderator, this opens the possibility to have different types of moderators at the bottom, so that other neutron beams of different intensity, or spectral shape, with respect to the ones delivered by the top moderator, could be envisaged, adding additional scientific opportunities to the facility.

Development of a target imaging system for the European Spallation Source

At the European Spallation Source (ESS) neutrons will be produced by a proton beam impinging on a rotating target wheel. The technology of the target wheel, which comprises a large number of closely spaced tungsten bricks and is cooled by helium, is largely untested. The durability of the target wheel and hence the overall ESS neutronic performance depend on the integrity of the tungsten bricks. In order to monitor whether the target geometry is preserved over the expected 5 year lifetime of the target wheel, we propose a Target Imaging System (TIS). The TIS consists of a scintillator array detecting the collimated single photon emission (decay gammas) from the activated tungsten bricks. Preliminary Monte Carlo simulations support the feasibility of this imaging system. As a proof-of-principle, an experimental test-rig is being constructed allowing to test the main aspects of the imaging system under conditions relevant to ESS.

In-Pile 4He Source for UCN Production at the ESS

ESS will be a premier neutron source facility. Unprecedented neutron beam intensities are ensured by spallation reactions of a 5 MW, 2.0 GeV proton beam impinging on a tungsten target equipped with advanced moderators. The work presented here aims at investigating possibilities for installing an ultra cold neutron (UCN) source at the ESS. One consequence of using the recently proposed flat moderators is that they take up less space than the moderators originally foreseen and thus leave more freedom to design a UCN source, close to the spallation hotspot. One of the options studied is to place a large 4He UCN source in a through-going tube which penetrates the shielding below the target. First calculations of neutron flux available for UCN production are given, along with heat-load estimates. It is estimated that the flux can give rise to a UCN production at a rate of up to  UCN/s. A production in this range potentially allows for a number of UCN experiments to be carried out at unprecedented precision, including, for example, quantum gravitational spectroscopy with UCNs which rely on high phase-space density.